Resistor and method of making the same



1940- E. R. STOEKLE ET AL 2,188,667

RESISTOR AND METHOD OF MAKING THE SAIE Filed Jan. 8, 1935 lllllllllllllllll I m M MW 5 Z'rzz/z'n Gear e 257216715 PatentedJan. 30, 1940PATENT OFFICE RESISTOR AND. METHOD OF MAKING THE SAME Erwin B. Stoekleand George M. Ehlers, Mllwaukee, Wis., assignors, by mesne assignments,V to Globe-Union Inc., Milwaukee, Wis., a corporation of DelawareApplication January 8, 1935, Serial No. 890

6 Claims. (Cl. 201-78) This invention relates to improvements inelectrical resistors of the so-called ceramic composition type, in whichan electrically conducting material is incorporated with an insulatingce- 6 ramic material to make resistors of any desired resistance value.

Our present invention is particularly applicable to fixed resistors ofthe character disclosed in the co-pending applications of one of the 10applicants hereof, George M. Ehlers, Serial No. 700,872, filed December4, 1933, now Patent No. 2,084,840, granted June 22, 1937; and Serial No.

746,840, filed October 1, 1934, the latter being adivision of the formerand covering the extrudll ing method of and apparatus for making suchresistors.

We have found that the usual method of incorporating conducting materialin.the composition comprising the resistance in the form of finelyground particles, does not give as satisfactory characteristics withrespect to its so-called voltage co-efilcient or change of resistancewith voltage, as does the resistor of our present invention. We havealso found that the minute 35 changes of resistance when the resistorcarries currents which evidence themselves as noises in the speaker of aradio receiver in which the resistance is used, have been substantiallydecreased in our improved form of resistor. The

noise so originating is referred to in the art as the noisecharacteristic of the resistor.

The composition resistors heretofore used vary in their so-calledvoltage co-eificient and noise characteristic a great deal, depending onhow 85 finely divided and how stably held is the electrically conductingmaterial incorporated in the composition of the resistance. In theresistors of the Ehlers applications, aforesaid, the conducting materialis very firmly and stably held in 40 a vitrified ceramic core enclosedin and surrounded by a ceramic jacket. In addition to firmly fixing theconducting material in the ceramic core, our experiments have shown thatfurther improvements can be made in the noise 4| characteristic of thesecomposition resistors by the method of incorporating the conductingmaterlal into the composition which is the subject of our presentinvention;

Resistors manufactured prior to our present so invention are composed ofconducting particles separated by insulating particles, the high valueof the resistance resulting to a large extent from the contactresistance between adjacent contacting particles. Since such contactingconducting particles. especially when of carbon, have thecharacteristics of a microphonic contact, conditions are favorable forcreating variations in the contact resistance of such microphoniccontacts which we believe is a principal source of the unfavorable noisecharacteristics of most composi- 5 tion resistors as heretofore placedon the market.

This thought is further confirmed by the fact that in the resistorsheretofore available, the noise characteristics become progressivelyworse as the resistances increase. This is to be expected, since most ofthe resistance in existing resistors of high value comprises contactresistance between adjacent particles. The resistors of our presentinvention and their method of manufacture are new and substantialdepartures from the previous composition resistors and their methods ofmanufacture, having reference here to resistors in which the contactingparticles are embedded in an insulating matrix. Our new resistorprovides a continuous, but tortuous' conducting path for the electriccurrent through the resistor instead of permitting the current to passfrom conducting particle to conducting particle as in the previousforms. In the Ehlers applications aforesaid, it is pointed out that hisresistance core includes a relatively coarse grained refractory materialand a fine grained plastic material to cement the coarse grainedparticles together. The cementing together of these coarse grainedparticles is accomplished when the vitrifying temperature causes thefinegrained ceramic to flux. After firing, the core of the Ehlersresistors comprises a porous mass, the pores appearing between thecoarse grains of the refractory ceramic and being filled with finelypowdered carbon or graphite, held there under compression by theshrinking of the ceramic during firing and the cooling after firing.This firm holding of the carbon particles in the Ehlers core makes thisresistor a. relatively stable and quiet one. We have, however, found byexperiments that improvements in quietness and other characteristics ofthis type of resistor may be accomplished by introducing the carbonconducting material in a different form, which we believe substantiallyeliminates contact resistance between particles of the carbon.

It is obvious that in order to get a continuous path for the electricalcurrent between the terminals of the Ehlers resistors, adjacent pores inthe porous core of the resistor must communicate with one another. Nowif in this porous or the walls of the pores and passages coated with acontinuous conducting medium, we would have the desired continuousresistance path free from all contact resistance such as might give riseto microphonic noises.

vWe have found an excellent method of accomplishing this result byintroducing into the core, carbon as a conducting material in the formof a hydrocarbon, such as pitch, resin, or similar material carbonizableat high temperatures. This carbonizable material is ground with thefiner ceramic of the core to practically colloidal fineness, and it isthen intimately mixed with the coarser refractory material composing thecore. The core and the jacket materials, in accordance with the methoddisclosed in the Ehlers applications, are simultaneously extruded from aforming die into the form of rods with the core material enclosed by thejacket material. Up to this point in the process the hydrocarbon whichlater yields carbon is not an elect'rical conductor.

, The dried rods, in accordance with our invention, are first subjectedto a temperature of 300 to 700 degrees Fahrenheit, depending upon thedesired resistance value of the finished resistor. Careful regulation ofthis first temperature governs the resistance of the finished resistorover a wide range of resistance and furnishes an excellent control overthe resistance of the finished product. This first temperature treatmentgoverns the distribution of the hydrocarbon in the core, since the pitchused melts and reaches various degrees of fluidity in this temperaturerange, and also certain of the more .volatile constituents thereby passoff in vapor.

The carbon is thus dispersed throughout the communicating interstices ofthe core in a nascent state and it is maintained in such dispersion bythe fusion of the plastic ceramic. The carbon is in a nascent state inthe sense that it is converted while in the mixture from a hydrocarbonsuch as resin, pitch, or the equivalent thereof, into carbon and itsdispersion is a combined result of the initial grinding and mixing ofthe ceramic and hydrocarbon, the presence of'the coarser grainedrefractory, and the subsequent melting and conversion of thehydrocarbon. The resistance rod is then brought to the fusiontemperature of its glazed coating and to the vitrifying temperatureofits ceramic jacket, which seals the core from ambient gases. At

this temperature, which is about 2300 degrees Fahrenheit, thehydrocarbon remaining in the core is completely carbonized depositingits car bon in the cavities or interstices of the core and theirconnecting passages. Thus a continu- 'ous, electrically conducting pathof hard carbon is One form of resistor made in accordance with ourinvention is shown in the accompanying drawing, in which:

Figure 1 is a perspective view partly in section, of a finishedresistor;

Fig. 2 is a fragmentary longitudinal sectional view taken through oneend of the resistor to a show its internal structure on a greatlymagnified or enlarged scale, the showing following that as depicted in amicro-photograph of a similar amass? section of a finished resistor madein accordance with our invention; and

Fig. 3 shows the extruding method and die for making the resistor.

as shown in the drawing, the resistor is in rod-like form, comprising acore i and a surrounding ceramic jacket 2. The core and the jacket areco-extensive in length and terminals 3, 3 of electrically conductingmaterial are applied to the ends of the jacket and in electrical contactwith the ends of the core. These terminals may be in the form of metalcaps, sprayed on the ends of the resistor, and lead wires 4, 4 forconnecting the resistor in an electrical circuit may be soldered to thecaps, as in the Ehlers applications. The solder coatings on the caps forsecuring the lead wires thereto are indicated at 5, 5.

- In extruding the resistor from a. forming die, as shown in Fig. 3, thecore material is surrounded by the jacket material. The rod as it leavesthe die is coated on its outer side with a coating of enamel frit,marked 6 in Fig. 3. The nozzles for applying this coating are marked 1,I. The concentric nozzles of the die are marked 8, 9. The core and thejacket materials are simultaneously extruded from the nozzles, the corematerial from the inner nozzle and the jacket material from the outernozzle. The extruded rods are cut to the desired lengths and thereupondried and fired as heretofore described.

Fig. 2 shows the core structure on a very much enlarged scale. Thefigure follows the showing in a micro-photograph of a longitudinalsection through one end of a finished resistor constructed in accordancewith our invention. The metal terminal sprayed on the end of theresistor is shown at 3. The ceramic jacket surrounding and protectingthe core is shown at 2, and the approximate boundary line between thecore and the jacket is shown at In. The ceramic material of the jacketis vitrified into an integral mass in the firing of the resistor andthis mass is integrally connected with the fusible ceramic particles inthe core mixture to connect the jacket and the coretogether. Theirregularly shaped coarse particles III of the core are the insulatingrefractory distributed through the core. The larger particles are fusedtogether by the fine ceramic fiuxing material with which the carbon ismixed. The carbon, indicated at H in Fig. 2, is deposited in the spacesor interstices and their connecting passages between and about thecoarse particles and provides a continuous, but tortuous path for theelectric current through the core. This current enters the core from themetal terminal 3. The current follows a tortuous path around the coarseparticles it of insulating material. The various passages between theinsulating particles shown in Fig. 2 are magnified about times. They areonly a few thousandths of an inch wide in the actual specimen. Theseminute passages form a continuous tortuous path for the current. Thehigher the proportion of coarse refractory incorporated in the core, thesmaller will become the passages between adjacent particles, andtherefore the higher will become the resistance of the core. The carbonH is deposited in these passages in the process of making the resistoras heretofore described. The period of time for each heating to whichthe extruded rods are subjected is such as to give the results hereindescribed.

Another consideration of particular importance I with respect to ourinvention is the ability to adjust such of the finished resistors ashappened to fall above the required resistance range. This isaccomplished by heating the completely fired resistor in vacuo to asufiicient temperature to-expel all the air from the pores in the core.As previously described the finished resistors after vitrification ha ean impervious ceramic jacket. However, the ends of the core of theresistor are not covered by the jacket but are covered by the metal capswhich are sprayed on the ends of the resistor. The portions of thesemetal caps which overlie the ends of the core are sufiiciently porous toallow the vacuum and temperature just referred toto operate throughoutthe core. The resistors so treated are then immersed in a liquidhydrocarbon or other carbonizable compound, which is drawn into thepores of the core through the portions of the sprayed metal end plateswhich overlie the ends of the core and which may be then carbonized byheating the resistor to the required temperature which will lower itsvalue.

It is to be understood that other electrical conducting materials may beused, such as silicon or metals which may be introduced during themixing of the core material in the form of compounds which arethermo-chemically converted into their component metals during thefiring of the resistor.

When desired, the electrical characteristics of our resistor such astemperature co-efiicient of resistance, may be modified by including inthe ceramic mixture for the conducting path, two elements such as carbonand a metal, each derivable from the thermo-decomposition 01' a suitablecompound or from a thermo-chemical interaction of two compounds such asa metallic compound and a carbon yielding compound.

It is to be understood, that the details herein described with respectto the method and the resistor may be variously changed and modifiedwithout departing from the spirit and scope of our invention except aspointed out in the annexed claims.

We claim as our invention:

1. The method of making an electrical resistor which comprises mixingwith a fine grained plastic ceramic a portion of a compound convertibleby heat into an electrical conductor, adding to said mixture a coarsegrained refractory ceramic, mixing the whole into a uniform plasticmass, extruding said mass into rods, drying said rods and firing them atthe maturing temperature of said ceramic mixture.

2. The method of making an electrical resistor which comprises mixingwith a fine grained plastic ceramic a carbonizable compound, adding tosaid mixture a relatively coarse grained refractory, intimately mixingthe same to form a plastic ceramic mixture, extruding said mixture intorods, drying said rods, and firing them at the maturing temperature ofsaid ceramic mixture during which firing said carbonizable compound isdecomposed to form a multiplicity of minute conducting paths of carbonbetween the grains of said coarse grained refractory.

3. The method of manufacturing an electrical resistor characterized byforming a core of ceramic material in which is included a compoundthermally convertible into an electrical conductor, surrounding saidcore with a protective insulating ceramic jacket simultaneously formedwith said core, and firing the formed and dried resistor into a strongintegral unit whereby the conductor yielding compound is retained insaid core to form conducting paths therethrough and both compound andconductor are protected from the deleterious effects of ambient gasesduring the firing process.

4. The hereindescribed method of making an electrical resistorcomprising grinding to approximately colloidal fineness a fine grainedplastic ceramic and an initially non-conductive hydrocarbon, intimatelymixing therewith a coarser grained refractory material, heating theresulting mixture to disperse the hydrocarbon and convert it into carbonand fix the value of the resistance and then rapidly raising thetemperature of the material to the vitrifying point of the ceramic.

5. The hereindescribed method of making an electrical resistorcomprising grinding to approximately colloidal fineness a fine grainedplastic ceramic and an initially non-conductive hydrocarbon, intimatelymixing therewith a coarser grained refractory, extruding the resultingmixture and an insulating ceramic into rods with the mixture the coreand the ceramic the jacket of the rods, drying the rods, heating therods up to a temperature to melt and distribute the hydrocarbon, convertit into carbon and fix the value of the resistance thereof, and thensubjecting the rods to a vitrifying temperature.

6. The method of controlling the desired resistance value of ceramicresistors including a thermally convertible conductor, yieldingcompound, which consists in first heating the resistor for a controlledperiod of time at a temperature below the vitrifying temperature of theceramic, in order to distribute and volatilize off the desired amount ofconductor yielding compound, then rapidly raising the temperature to thevitrifying point of the ceramic whereby the remaining containedconductor yielding compound is sealed within said resistor and isthermally converted into electrically conducting material within thebody of the resistor.

ERWIN R. STOEKLE.

GEORGE M. EHLERS.

